ccna study guide by sybex phần 5 pdf

Routing Information Protocol RIP 263RIP Timers RIP uses three different kinds of timers to regulate its performance: Route update timer Sets the interval typically 30 seconds between pe

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Routing Information Protocol (RIP) 263

RIP Timers

RIP uses three different kinds of timers to regulate its performance:

Route update timer Sets the interval (typically 30 seconds) between

periodic routing updates, in which the router sends a complete copy of its routing table out to all neighbors

Route invalid timer Determines the length of time that must expire (90

seconds) before a router determines that a route has become invalid It will come to this conclusion if it hasn’t heard any updates about a partic-ular route for that period When that happens, the router will send out updates to all its neighbors letting them know that the route is invalid

Route flush timer Sets the time between a route becoming invalid and

its removal from the routing table (240 seconds) Before it is removed from the table, the router notifies its neighbors of that route’s impending doom The value of the route invalid timer must be less than that of the route flush timer This is to provide the router with enough time to tell its neighbors about the invalid route before the routing table is updated

Configuring RIP Routing

To configure RIP routing, just turn on the protocol with the router rip command and tell the RIP routing protocol which networks to advertise That’s it As an example, let’s configure our four-router internetwork with RIP routing

2621A

RIP has an administrative distance of 120 Static routes have an administrative distance of 1 by default and, since you currently have static routes configured, the routing tables won’t be propagated with RIP information The first thing you need to do is to delete the static routes off each router This is done with the no ip route command Notice that in the 2621A router output below you must type the whole ip route command to delete the entry

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to advertise Notice that in the router configuration below the routing tocol is not told which subnets to advertise; it is told the classful boundary RIP will find the subnets and advertise them

pro-2621A(config)#router rip 2621A(config-router)#network 172.16.0.0 2621A(config-router)#^Z

2621A#

That’s it Two commands, and you’re done—sure makes your job a lot easier than when using static routes, doesn’t it? However, keep in mind the extra router CPU process and bandwidth that you’re consuming

2501A

To configure RIP on the 2501A router, you need to remove the three static routes you added from the earlier example Once you make sure no routes are in the routing table with a better administrative distance than 120, you can add RIP Again, if you do not remove the static routes, RIP routes will never be used on the router

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RouterC(config-router)#network 172.16.0.0

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266 Chapter 5 IP Routing

It is important to remember why we are doing this Directly connected routes have an administrative distance of 0, static routes have an adminis-trative distance of 1, and RIP has an administrative distance of 120 I call RIP the gossip protocol because it reminds me of junior high school, where if you hear a rumor, it must be true That’s how RIP behaves on an internetwork—exactly like my 14-year-old son

Verifying the RIP Routing Tables

Each routing table should now have the routers’ directly connected routes as well as RIP-injected routes received from neighbor routers

The router output below shows the contents of the 2621A routing table

The next router output displays the routing table of the 2501A routers

2501A#sh ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M – [output cut]Gateway of last resort is not set

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R 172.16.50.0 [120/2] via 172.16.20.2, 00:00:11, Serial0

R 172.16.40.0 [120/1] via 172.16.20.2, 00:00:11, Serial0

R 172.16.30.0 [120/1] via 172.16.20.2, 00:00:11, Serial0

C 172.16.20.0 is directly connected, Serial0

C 172.16.10.0 is directly connected, Ethernet0

R 172.16.30.0 [120/1] via 172.16.40.1, 00:00:06, Serial0

R 172.16.20.0 [120/1] via 172.16.40.1, 00:00:06, Serial0

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RIP has worked well in our little internetwork However, since this nique has a maximum hop count of only 15 hops (where 16 is deemed unreachable) and performs full routing-table updates every 30 seconds, it can cause havoc on a larger internetwork

tech-Holding Down RIP Propagations

You may not want your RIP network advertised everywhere on your LAN and WAN For instance, there is no advantage to advertising your RIP net-work to the Internet

There are a few different ways to stop unwanted RIP updates from agating across your LANs and WANs The easiest way to do this is through the passive-interface command This command prevents RIP update broadcasts from being sent out a defined interface, but that same interface can still receive RIP updates

prop-The following is an example of how to configure a passive-interface

on a router:

RouterA#config t RouterA(config)#router rip RouterA(config-router)#network 10.0.0.0 RouterA(config-router)#passive-interface serial 0

The above command will stop RIP updates from being propagated out serial interface 0, but serial interface 0 can still receive RIP updates

Interior Gateway Routing Protocol (IGRP)

Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary distance-vector routing protocol This means that all your routers must be Cisco routers to use IGRP in your network Cisco created this routing pro-tocol to overcome the problems associated with RIP

IGRP has a maximum hop count of 255 with a default of 100 This is helpful in larger networks and solves the problem of there being only 15

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Interior Gateway Routing Protocol (IGRP) 269

metric Reliability, load, and Maximum Transmission Unit (MTU) can also

be used, although they are not used by default

IGRP Timers

To control performance, IGRP includes the following timers with default settings:

Update timers These specify how frequently routing-update messages

should be sent The default is 90 seconds

Invalid timers These specify how long a router should wait before

declaring a route invalid if it doesn’t receive a specific update about it The default is three times the update period

Holddown timers These specify the holddown period The default is

three times the update timer period plus 10 seconds

Flush timers These indicate how much time should pass before a route

should be flushed from the routing table The default is seven times the routing update period

Configuring IGRP Routing

The command used to configure IGRP is the same as the one used to ure RIP routing with one important difference: you use an autonomous sys-tem (AS) number All routers within an autonomous system must use the same AS number, or they will not communicate with routing information Here is an example of how to turn on IGRP routing:

config-RouterA#config t RouterA(config)#router igrp 10 RouterA(config-router)#network 172.16.0.0

Notice that the configuration in the above router commands is as simple

as in RIP routing except that IGRP uses an AS number This number tises only to routers you want to share routing information with

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dis-The router output below shows the options available under the router igrp as command prompt

The load balancing and traffic sharing are covered more in depth in Sybex’s

CCNP: Routing Study Guide.

Configuring IGRP in Our Internetwork

Configuring IGRP is pretty straightforward and not much different from configuring RIP You do need to decide on an AS number before you configure your routers Remember that all routers in your internetwork must use the same AS number if you want them to share routing information In our internetwork, we’ll use AS 10 to configure the routers

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Configuring IGRP in Our Internetwork 271

2621A#

The router igrp command turns IGRP routing on in the router As with RIP, you still need to add the network number you want to advertise IGRP uses classful routing, which means that subnet mask information is not sent with the routing protocol updates

2501A#

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Verifying the IGRP Routing Tables

Once the routers are configured, you need to verify the configuration with the show ip route command

In all of the following router outputs, notice that the only routes to works are either directly connected or IGRP-injected routes Since we did not turn off RIP, it is still running in the background and taking up both router CPU cycles and bandwidth However, the routing tables will never use a RIP-found route because IGRP has a better administrative distance than RIP does

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net-Verifying the IGRP Routing Tables 273

The router output below is from the 2621A router Notice that all routes are in the routing table

Codes: C - connected, S - static, I - IGRP, R - RIP, M –

[output cut]

T - traffic engineered route

Gateway of last resort is not set

C 172.16.10.0 is directly connected, FastEthernet0/0

The I means IGRP-injected routes The [100/160360] is the tive distance of IGRP and the composite metric The lower the composite metric, the better the route

administra-The following router output shows the routing table for the

2501A router

[output cut]

U - per-user static route, o - ODR

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172.16.0.0/24 is subnetted, 5 subnets

I 172.16.50.0 [100/8576] via 172.16.40.2, 00:01:11, Serial1

I 172.16.10.0 [100/158350] via 172.16.20.1, 00:00:36, Serial02501B#

The following router output shows the 2501C routing table

2501C#sh ip route

[output cut]

172.16.0.0/24 is subnetted, 5 subnets

I 172.16.30.0 [100/8576] via 172.16.40.1, 00:00:28, Serial0

I 172.16.20.0 [100/160250] via 172.16.40.1, 00:00:28, Serial0

I 172.16.10.0 [100/160350] via 172.16.40.1, 00:00:28, Serial02501C#

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Verifying Your Configurations 275

Verifying Your Configurations

It is important to verify your configurations once you have completed

them, or at least, once you think you have completed them The following

list includes the commands you can use to verify the routed and routing tocols configured on your Cisco routers The first command is covered in the previous section; the others are covered in upcoming sections

pro- show ip route

show protocols

show ip protocol

debug ip rip

debug ip igrp events

debug ip igrp transactions

The Show Protocols Command

The show protocols command is useful because it shows you the Network layer addresses configured on each interface

2501B#sh protocol

Global values:

Internet Protocol routing is enabledEthernet0 is up, line protocol is up Internet address is 172.16.30.1/24Serial0 is up, line protocol is up Internet address is 172.16.20.2/24Serial1 is up, line protocol is up Internet address is 172.16.40.1/242501B#

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The Show IP Protocol Command

The show ip protocol command shows you the routing protocols that are configured on your router Notice in the following output that both RIP and IGRP are running on the router, but only IGRP appears in the routing table because of its lower administrative distance

The show ip protocols command also displays the timers used in the routing protocol Notice in the output below that RIP is sending updates every 30 seconds, which is the default Notice further down that RIP is rout-ing for network 172.16.0.0, and the two neighbors it found are 172.16.40.2 and 172.16.20.1

2501B#sh ip protocol

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 6 seconds Invalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Redistributing: rip

Default version control: send version 1, receive any version

Interface Send Recv Key-chain Ethernet0 1 1 2

Serial0 1 1 2 Serial1 1 1 2 Routing for Networks:

172.16.0.0 Routing Information Sources:

Gateway Distance Last Update 172.16.40.2 120 00:00:21 172.16.20.1 120 00:00:23 Distance: (default is 120)

In the preceding router output, the last entry is the default administrative distance for RIP (120)

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Verifying Your Configurations 277

The router output below shows the IGRP routing information The default update timer is 90 seconds by default, and the administrative dis-tance is 100

Routing Protocol is "igrp 10"

Sending updates every 90 seconds, next due in 42 seconds Invalid after 270 seconds, hold down 280, flushed after 630

Outgoing update filter list for all interfaces is

Incoming update filter list for all interfaces is

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

IGRP maximum hopcount 100

IGRP maximum metric variance 1

Redistributing: eigrp 10, igrp 10

Routing for Networks:

172.16.0.0

Routing Information Sources:

Gateway Distance Last Update

The invalid timer is set at 270 seconds; it is three times the update timer

If a router update is not received in three update periods, the route is sidered invalid The holddown timer is 280, which is around three times the update timer This is the number of seconds a route is suppressed while wait-ing for a new update to be received If a new update is not received before the holddown timer expires, the flush timer will start When the flush timer

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con-278 Chapter 5 IP Routing

The Debug IP RIP Command

The debug ip rip command sends routing updates as they are sent and received on the router to the console session If you are telnetted into the router, you’ll need to type the command terminal monitor to be able to receive the output from the debug commands

Notice in the following router output that RIP is both sent and received

on serial 1, serial 0, and Ethernet 0 interfaces This is a great troubleshooting tool The metric is the hop count

07:12:56: 172.16.10.0 in 1 hops07:12:58: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (172.16.30.1)

07:12:58: subnet 172.16.40.0, metric 107:12:58: subnet 172.16.20.0, metric 107:12:58: RIP: sending v1 update to 255.255.255.255 via Serial0 (172.16.20.2)

07:12:58: subnet 172.16.40.0, metric 107:12:58: subnet 172.16.30.0, metric 107:12:58: RIP: sending v1 update to 255.255.255.255 via Serial1 (172.16.40.1)

07:12:58: subnet 172.16.30.0, metric 107:12:58: subnet 172.16.20.0, metric 1

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com-Verifying Your Configurations 279

The Debug IP IGRP Command

With the debug ip igrp command, there are two options, events and transactions, as shown in the router output below:

2501B#debug ip igrp ?

events IGRP protocol events transactions IGRP protocol transactionsThe difference between these commands is explained in the following sections

The Debug IP IGRP Events Command

The debug ip igrp events command is a summary of the IGRP routing information that is running on the network The following router output shows the source and destination of each update as well as the number of routers in each update Information about individual routes is not generated with this command

2501B#debug ip igrp events

IGRP event debugging is on07:13:50: IGRP: received request from 172.16.40.2 on Serial1

07:13:50: IGRP: sending update to 172.16.40.2 via Serial1 (172.16.40.1)

07:13:51: IGRP: Update contains 3 interior, 0 system, and

0 exterior routes

07:13:51: IGRP: Total routes in update: 307:13:51: IGRP: received update from 172.16.40.2 on Serial1

07:13:51: IGRP: Update contains 1 interior, 0 system, and

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The Debug IP IGRP Transactions Command

The debug ip igrp transactions command shows message requests from neighbor routers asking for an update and the broadcasts sent from your router towards that neighbor router

In the following output, a request was received from a neighbor router on network 172.16.40.2 to serial 1 of 2501B The 2501B router responded with

an update packet

2501B#debug ip igrp transactions

IGRP protocol debugging is on07:14:05: IGRP: received request from 172.16.40.2 on Serial1

07:14:05: IGRP: sending update to 172.16.40.2 via Serial1 (172.16.40.1)

07:14:05: subnet 172.16.30.0, metric=110007:14:05: subnet 172.16.20.0, metric=15825007:14:05: subnet 172.16.10.0, metric=15835007:14:06: IGRP: received update from 172.16.40.2 on Serial1

07:14:06: subnet 172.16.50.0, metric 8576 (neighbor 1100)

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internet- Default routing and how default routing can be used in stub networks

Dynamic routing and how to solve loops in distance-vector routing protocols

Configuring and verifying RIP routing

Configuring and verifying IGRP routing

Key Terms

Before you take the exam, be sure you’re familiar with the following terms:

classless routing composite metric holddown hop count poison reverse updates route poisoning split horizon

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Commands in This Chapter

show ip route Displays the IP routing table

IP route Creates static and default routes

on a router

IP classless Is a global configuration command

used to tell a router to forward packets to a default route when the destination network is not in the routing table

router RIP Turns on IP RIP routing on a

routernetwork Tells the routing protocol what

network to advertise

No IP route Removes a static or default routerouter igrp as Turns on IP IGRP routing on a

router

between the best metric and the worst acceptable metric

traffic-share balanced Tells the IGRP routing protocol to

share links inversely proportional

to the metrics traffic-share min Tells the IGRP routing process to

use routes that have only minimum costs

show protocols Shows the routed protocols and

network addresses configured on each interface

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Commands in This Chapter 283

show ip protocols Shows the routing protocols

and timers associated with each routing protocol configured on

a routerdebug ip rip Sends console messages displaying

information about RIP packets being sent and received on a router interface

debug ip igrp events Provides a summary of the IGRP

routing information running on the network

debug ip igrp transactions Shows message requests from

neighbor routers asking for an update and the broadcasts sent from your router to that neighbor router

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Written Lab

Write the answers to the following questions

1. Create a static route to network 172.16.10.0/24 with a next hop way of 172.16.20.1 and an administrative distance of 150

gate-2. Write the commands used to turn RIP routing on in a router and advertise network 10.0.0.0

3. Write the commands to stop a router from propagating RIP tion out serial 1

informa-4. Write the commands to create an AS 10 with IGRP in your 172.16.0.0 network

5. Write the commands to configure a default route on a router to go to 172.16.50.3

6. What works with triggered updates to help stop routing loops in tance-vector networks?

dis-7. What stops routing loops in distance-vector networks by sending out

a maximum hop count as soon as a link fails?

8. What stops routing loops in distance-vector networks by not ing information learned on an interface out that same interface?

resend-9. The command controls the load balancing between the best metric and the worst acceptable metric

10. What command is used to send routing updates as they are sent and received on the router to the console session?

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Hands-on Labs 285

Hands-on Labs

In the following hands-on labs, you will configure a network with three

2501 routers and one 2621 router

The following labs will be covered:

Lab 5.1: Creating Static RoutesLab 5.2: Dynamic Routing with RIPLab 5.3: Dynamic Routing with IGRPFigure 5.9 will be used to configure all routers

F I G U R E 5 9 Hands-on lab internetwork

Lab 5.1: Creating Static Routes

In this first lab, you will create a static route in all four routers so that the routers see all networks Verify with the Ping program when complete

1. The 2621 router is connected to network 172.16.10.0/24 It does not know about networks 172.16.20.0/24, 172.16.30.0/24, 172.16.40.0/24, and 172.16.50.0/24 Create static routes so that the 2621 router can see all networks, as shown here

2501A F0/0

2621A

S1 S0

E0

2501B

E0 S0

2501C

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2. Save the current configuration for the 2621 router by going to the

enabled mode, typing copy run start, and pressing Enter.

3. On Router A, create a static route to see networks 172.16.10.0/24, 172.16.30.0/24, 172.16.40.0/24, and 172.16.50.0/24, as shown here

RouterA#config t RouterA(config)#ip route 172.16.30.0 255.255.255.0

4. Save the current configuration for Router A by going to the enabled

mode, typing copy run start, and pressing Enter.

5. On Router B, create a static route to see networks 172.16.10.0/24 and 172.16.50.0/24, which are not directly connected Create static routes

so that Router B can see all networks, as shown here

RouterB#config t RouterB(config)#ip route 172.16.10.0 255.255.255.0

172.16.20.1

RouterB(config)#ip route 172.16.50.0 255.255.255.0

172.16.40.2

The first command told Router B that to get to network 172.16.10.0/24,

it needs to use 172.16.20.1 The next command told Router B to get

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Hands-on Labs 287

6. Router C is connected to networks 172.16.50.0/24 and 172.16.40.0/

24 It does not know about networks 172.16.30.0/24, 172.16.20.0/

24, and 172.16.10.0/24 Create static routes so that Router C can see all networks, as shown here

RouterC#config t RouterC(config)#ip route 172.16.30.0 255.255.255.0

Save the current configuration for Router C by going to the enable

mode, typing copy run start, and pressing Enter.

Now ping from each router to your hosts and from each router to each router If it is set up correctly, it will work

Lab 5.2: Dynamic Routing with RIP

In this lab, we will use the dynamic routing protocol RIP instead of static and default routing

1. Remove any static routes or default routes configured on your routers

by using the no ip route command For example:

RouterA#config t RouterA(config)#no ip route 172.16.10.0

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2. After your static and default routers are clear, go into configuration

mode on Router A by typing config t.

3 Tell your router to use RIP routing by typing router rip and pressing

Enter, as shown here:

config t router rip

4 Add the network number you want to advertise by typing network

172.16.0.0 and pressing Enter.

5. Press Ctrl+Z to get out of configuration mode

6. Go to Routers B and C and the 2621 router and type the same mands, as shown here:

com-Config t Router rip Network 172.16.0.0

7. Verify that RIP is running at each router by typing the following mands at each router:

com-show ip protocol show ip route show running-config or show run

8 Save your configurations by typing copy run start or copy

running-config startup-running-config and pressing Enter at each router

9. Verify the network by pinging all remote networks and hosts

Lab 5.3: Dynamic Routing with IGRP

In this lab, you will run the IGRP routing protocol simultaneously with RIP routing

1 Log into your routers and go into privileged mode by typing en or

enable.

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3 From the configuration mode on Router A, type router igrp ?.

4. Notice that it asks for an autonomous system number This is used to

allow only routers with the same AS number to communicate Type 10

and press Enter Your router can be configured to be part of as many different ASs as necessary

5 At the config-router prompt, type network 172.16.0.0 Notice that we

add the classful network boundary to advertise rather than the subnet numbers

6 Press Ctrl+Z to get out of configuration mode

7. Go to Routers B and C and the 2621 router and type the commands shown here:

RouterB(config)#router igrp 10 RouterB(config-router)#network 172.16.0.0

8. Verify that IGRP is running by typing the following command at each router:

show ip protocol

Notice that this shows you your RIP and IGRP routing protocols and the update timers

sh ip route

This should let you see all eight subnets: 10, 11, 15, 20, 30, 40, 50, and

55 Some will be directly connected, and some will be I routes, which are IGRP-injected routes RIP is still running, but if you look at the routing table, you’ll notice the network entry has a network number

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weight, of the route that is used to determine the best path to a network

show running-config

This lets you see that RIP and IGRP are configured

9 To save your configurations, type copy running-config startup-config

or copy run start and press Enter at each router.

10. Verify the network by pinging all routers, switches, and hosts

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D Router(config-router)#no routing updates

6. What is the default routing metric used by IGRP? (Choose all that apply.)

E. Last hop available

8. What are holddowns used for?

A. To hold down the protocol from going to the next hop

B. To prevent regular update messages from reinstating a route that has gone down

C. To prevent regular update messages from reinstating a route that has just come up

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Review Questions 293

9. What is split horizon?

A. When a router differentiates on which interface a packet arrived and does not advertise that information back out the same inter-face

B. When you have a large bus (horizon) physical network, it splits the traffic

C. It holds the regular updates from broadcasting to a downed link

D. It prevents regular update messages from reinstating a route that has gone down

10. What is poison reverse?

A. It sends back the protocol received from a router as a poison pill, which stops the regular updates

B. It is information received from a router that can’t be sent back to the originating router

C. It prevents regular update messages from reinstating a route that has just come up

D. It describes when a router sets the metric for a downed link to infinity

11. What is the default administrative distance for IGRP?

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A. They send out partial updates every 60 seconds.

B. They send their complete routing table every 60 seconds

C. They send their entire routing table every 30 seconds

D. They update every 90 seconds

16. Which Cisco IOS command can you use to see the IP routing table?

A. sh ip config

B. sh ip arp

C. sh ip route

D. sh ip table

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Review Questions 295

17. What is the administrative distance used for in routing?

A. Determining the network administrator for entering that route

19. Which of the following is true about IP routing?

A. The destination IP address changes at each hop

B. The source IP address changes at each hop

C. The frame does not change at each hop

D. The frame changes at each hop

20. Which of the following is true when creating static routes? (Choose all that apply.)

A. The mask parameter is optional

B. The gateway parameter is required

C. The administrative distance is required

D. The administrative distance is optional

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Answers to the Written Lab

1. ip route 172.16.10.0 255.255.255.0 172.16.20.1 150

2. config trouter ripnetwork 10.0.0.0

3. config trouter rippassive-interface serial 1

4. config trouter igrp 10network 172.16.0.0

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Answers to Review Questions 297

Answers to Review Questions

1. D RIP uses the distance-vector routing algorithm and uses only hop count as a metric to determine the pest path to an internetwork

2. D IGRP is Cisco’s proprietary distance-vector routing algorithm

3. B The command show ip protocol will show you the configured routing protocols on your router, which includes the timers

4. B RIP only uses hop count to determine the best path to a remote network

5. C The config-router passive-interface command stops updates from being sent out an interface

6. D, E Bandwidth and delay of the line are used by IGRP to determine the best way to a remote network

7. D RIP, by default, is only configured to run 15 hops; 16 is deemed unreachable

8. B Holddowns prevent regular update messages from reinstating a downed route

9. A Split horizon will not advertise a route back to the same router it learned the route from

10. D Poison reverse is used to communicate to a router that the router understands the link is down and that the hop count to that network

is set to infinity, or unreachable

11. B IGRP default administrative distance is 100; RIP’s default istrative distance is 120

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admin-298 Chapter 5 IP Routing

13. C OSPF (Open Shortest Path First) is a true link state IP routing tocol It uses only bandwidth as a way to determine the best path to a remote network

pro-14. A, D The commands show ip route and debug ip rip are used to support and verify RIP networks

15. C Distance-vector routing protocols send their complete routing table out all active interfaces RIP is every 30 seconds The best answer is C, because unlike D, it describes the entire routing table being broadcast

16. C The command show ip route shows the IP routing table, the ric used, and the interface used to find a remote network

met-17. C The administrative distance is used in routing to decide the worthiness of a route 0 is the highest rating

trust-18. C Statically connected routes are identified in the routing table with

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6

Virtual LANs (VLANs)

THE CCNA EXAM TOPICS COVERED IN THIS CHAPTER INCLUDE THE FOLLOWING:

Describe Virtual LANs

Describe Frame Tagging

Describe Inter-Switch Link Routing

Describe Virtual Trunking Protocol

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